1. Field of the Invention
The invention relates to an apparatus and method for tinting hydrophilic contact lenses. A hydrated finished lens is placed in a holding device constructed to mask areas of the lens that are not to be tinted. The holding device containing the lens is then placed in a dye bath and the lens is tinted using reactive or vat dyes. Any number of lenses can be tinted at one time.
2. Description of Related Art
Tinted contact lenses offer several advantages over clear contact lenses. Contact lenses can be tinted to enhance or even change the color of the iris, which is often desirable since contact lenses are primarily worn for cosmetic reasons. Tinting a lens can also serve as a location aid, making it easier to locate a decentered lens on the eye or to find a lens if it has been dropped, lost or misplaced. Tinted lenses reduce the annoying glare often experienced with clear lenses out of doors. There are also several therapeutic reasons to tint a lens. An opaque black lens can also be used as a sleep aid for people with sleeping problems. Lenses can be tinted to absorb ultra-violet light to prevent retinal damage after surgery. Tinted lenses also may be used to cover unsightly eye injuries. U.S. Pat. No. 3,586,423 describes a tinted lens that enables color-blind individuals to distinguish colors.
The first contact lenses made of polymethyl methacrylate (hard lenses) were tinted by dissolving or suspending dyes or pigments in the monomer or pre-polymer before polymerization. Since hard lenses are small (8.0-9.0 mm diameter) the entire lens can be tinted without being visible on the sclera. Soft (hydrophilic) lenses, however, are much larger (14 mm diameter) so that the entire lens cannot be tinted without making the eye appear strange. For a tinted lens to appear normal, the tinted zone cannot be larger than the iris (11 mm diameter). Thus, soft lenses must be tinted only in the central area (11 mm diameter) and the periphery must be left clear. Some practitioners also prefer to have the center (5 mm diameter) directly over the pupil left clear.
The first tinted soft lenses were made in the early 1970's by a number of workers in the United States and Europe. Two basic techniques were used. In one, the colorant was included in the polymer itself at formulation, while the other technique applied the colorant to a finished clear lens.
The first technique consisted of casting concentric rings of clear and tinted polymer in tubular molds. Pigments were used for coloring since water soluble dyes would extract on hydration. The resulting rods were machined into buttons and lenses using standard manufacturing techniques. After hydration and extraction, the finished lenses were ready for wear. This process had several problems that made it unattractive commercially. Incomplete bonding at the junctions of the concentric polymer rings frequently would cause lenses to tear at the junction on hydration. The expansion ratios of each zone had to match exactly or buckling would occur at the junctions on hydration. Pigments also have a large enough particle size to cause a decrease in fluid transport and thus decreases oxygen permeability.
The second technique applied oil-soluble dyes to a finished lens swollen in an organic solvent. Solvents used were miscible in water and organic solvents, for example: methanol, ethanol, dimethyl sulfoxide, dimethyl formamide, and tetrahydrofuran. In a typical protocol, a finished, hydrated soft lens would be placed in a swelling solvent until maximum expansion was achieved. The expanded lens was then placed on a template machined from an aluminum or brass block. The metal template was designed with exchangable masking dies that allowed for the tinting solution to be applied only to selected areas of the lens. A solution of an oil-soluble dye in the swelling solvent was then applied and allowed to diffuse into the lens matrix. After the desired color was achieved, excess dye was poured off, the lens was rinsed with solvent, and then reequilibrated in normal saline solution. The exchange from organic to aqueous solvent caused the dye to precipitate in the lens matrix since the dye would be insoluble in water.
This process has several serious problems that prevented it from being commercially viable. The expansion step is a very destructive process since it breaks the crosslinks that hold the polymer matrix together thus decreasing the already low durability of a lens. In the expanded state, the lens is very fragile and is easily damaged in handling, particularly when the masking die is placed over the lens on the template. The dyes themselves are prone to leaching from the lens since there is nothing holding the dyes in the matrix. The swelling solvents, which are toxic to ocular tissue, must be completely extracted from the lens on re-equilibration. The process itself is slow and not adaptable to volume production required to make it commercially successful.
There are two modifications of this second process in use today. All have in common a masking device to cover areas of a lens not to be tinted. All are one-at-a-time processes in that each lens is dyed individually. The process can be automated by placing the lens holders on a conveyor and injecting and removing dying, fixing, and rinsing solutions as the holders move along the line.
In the first modification, a vat dye is used as the colorant. The dye is applied in its soluble reduced form to the hydrated lens. After the dye has been absorbed into the lens, it is oxidized to its insoluble form. This traps the precipitated colorant in the polymer matrix. This is an improvement over the use of oil-soluble dyes since the lens does not have to be expanded in solvent. It has a similar problem, however, since the dyes are prone to leaching since there is nothing holding the dye in the matrix.
In the second modification, reactive dyes are used as colorants. The dye is applied in the same manner as a vat dye but is activated to cause it to chemically bond to the lens surface. This is an improvement since the colorant is permanent and will not leach from the lens.
Both modifications are still one lens at a time processes that have been automated. They are subject to mechanical breakdown and require considerable capital expense in set-up. They are not true bulk processes and offer little flexibility in production.
Other methods of tinting soft contact lenses include that described in British patent specification 1,004,424, dated May 31, 1963 which describes contact lenses made of a synthetic hydrocolloidal polymer which is at least partly colored by using a dye bath and a masking device. Also, British patent specification No. 1,163,617, dated Nov. 11, 1966 discloses a colored contact lens wherein a single-colored or multi-colored pattern is provided between at least two layers of a physiologically suitable hydrogel of which at least the first layer on a convex surface is transparent.
A recent method is that of U.S. Pat. No. 4,468,229, which discloses the covalent bonding of reactive dyestuffs to monomer units of the polymer backbone of polymeric lens material.
Accordingly, it is an the object of the present invention to provide an apparatus and a method for tinting soft contact lenses which offer the flexibility of creating any tinting pattern on any lens design and which is capable of tinting one or more lenses in a single operation.
These and other objects can be appreciated by reference to the following description of the invention and its preferred embodiments.